Viswanath Meenakshisundaram’s research while affiliated with Virginia Tech and other places

Additive manufacturing enables the creation of novel structures and geometries previously unattainable through traditional processing techniques. In particular, vat photopolymerization provides unprecedented resolution through the tailored delivery of light with photo-crosslinkable or photo-polymerizable materials. Traditionally, chemical crosslinks generate a permanent network, which exhibits swelling but not dissolution. In this work, photopolymerization of photo-reactive monomers with acrylate, acrylamide, and vinyl polymerizable sites enabled the formation of water-soluble 3D printed parts using vat photopolymerization. A library of monomers with varied ionic and hydrogen bonding sites provided photopolymerized films with tensile properties approaching 1200 % elongation at break and 0.47 MPa stress at 100 % elongation. The rate of polymerization and the subsequent mechanical properties revealed a dependence on the type of supramolecular interactions and functionality on the resulting hydrogel. The diverse functionality of the monomers enabled aqueous dissolution times ranging from 27 to 41 min. Vat photopolymerization of a trimethylammonium ethyl acrylate chloride solution and with 30 wt% N-vinyl pyrrolidone provided 3D parts with fine structural resolution. This method of creating soluble, water-swollen structures through vat photopolymerization provides future research with a larger library of monomers for diverse applications including soluble support scaffolds.

This work describes the first example of a hydrogenated polybutadiene elastomer photopolymer that addresses the process constraints of vat photopolymerization (VP) additive manufacturing. A synthetic method, which involves simultaneous thiol-ene step growth chain extension and acrylate crosslinking, addresses traditional challenges associated with this leading 3D printing platform. This facile, one-pot strategy combines the processing advantages of low molecular weight oligomers with the tunable thermomechanical and mechanical performance of higher molecular weight polymeric networks directly during printing, without requiring a post-processing step. The addition of photo-initiator to mixtures of liquid polybutadiene oligomer and miscible dithiols enabled selective photocuring under UV exposure to form high-strain, elastic parts in comparison to neat diacrylate systems. Photolithographic printing of these photopolymers enabled the fabrication of three-dimensional, hydrocarbon elastomer objects. Photorheology elucidated curing behavior as a function of composition and UV intensity, while optical imaging and SEM revealed quality and resolution.

Photocuring and vat photopolymerization (VP) additive manufacturing (AM) is reported for two families of fully amorphous poly(dimethyl siloxane) (PDMS) terpolymers containing either diphenylsiloxy (DiPhS) or diethylsiloxy (DiEtS) repeating units. A thiol‐functionalized PDMS crosslinker enables rapid crosslinking in air using efficient thiol–ene addition. Differential scanning calorimetry and dynamic mechanical analysis (DMA) confirm the absence of crystallinity for the DiPhS‐containing systems, while DMA shows a rubbery plateau extending to greater than 200 °C for the DiEtS‐containing system. VP‐AM of both photopolymer systems afford well‐defined 3D geometries, including high aspect ratio structures, which demonstrate feasibility of these photopolymers for the 3D printing of unique geometric objects that require elastomeric performance to temperatures as low as −120 °C. Vat photopolymerization additive manufacturing with vinyl‐functional siloxane terpolymers and a thiol‐functional siloxane, enable thiol–ene addition for rapid crosslinking in the presence of air. Thermal analysis confirms the absence of crystallinity, and dynamic mechanical analysis displays a rubbery plateau width of more than 200 °C. These photopolymer compositions hold promise for 3D printed elastomers that offer temperature‐independent moduli from room temperature to −120 °C.

Polyamic acid (PAA) salts are amenable to photocuring additive manufacturing processes of all-aromatic polyimides. Due to an all-aromatic structure, these high-performance polymers are exceptionally chemically and thermally stable but are not conventionally processable in their imidized form. The facile addition of 2-(dimethylamino)ethyl methacrylate (DMAEMA) to commercially available poly(4,4′-oxydiphenylene pyromellitamic acid) (PMDA-ODA PAA) afforded ultraviolet curable PAA salt solutions. These readily prepared solutions do not require multistep synthesis, exhibited fast gel times (<5 s), and rendered high G′ gel-state moduli. Vat photopolymerization 3D printing afforded self-supporting organogels. Subsequent thermal treatment rendered the cross-linked PAA precursor to all-aromatic PMDA-ODA polyimide. This fast and facile strategy makes PMDA-ODA polyimides accessible in three dimensions and offers impact on aerospace or automotive technologies.

A novel, poly(dimethyl siloxane)-based photopolymer that exhibits simultaneous linear chain extension and crosslinking was suitable for vat photopolymerization additive manufacturing. Photopolymer compositions consisted of dithiol and diacrylate functional poly(dimethyl siloxane) oligomers, where simultaneous thiol-ene coupling and free radical polymerization provided for linear chain extension and crosslinking, respectively. Compositions possessed low viscosity before printing and the modulus and tensile strain at break of a photocured, higher molecular weight precursor after printing. Photorheology and soxhlet extraction demonstrated highly efficient photocuring, revealing a calculated molecular weight between crosslinks of 12,600 g/mol and gel fractions in excess of 90% while employing significantly lower molecular weight precursors (i.e. < 5300 g/mol). These photocured objects demonstrated a 2× increase in tensile strain at break as compared to a photocured 5300 g/mol PDMS diacrylamide alone. These results are broadly applicable to the advanced manufacturing of objects requiring high elongation at break.

High-performance, all-aromatic, insoluble, engineering thermoplastic polyimides, such as pyromellitic dianhydride and 4,4'-oxydianiline (PMDA-ODA) (Kapton), exhibit exceptional thermal stability (up to ≈600 °C) and mechanical properties (Young's modulus exceeding 2 GPa). However, their thermal resistance, which is a consequence of the all-aromatic molecular structure, prohibits processing using conventional techniques. Previous reports describe an energy-intensive sintering technique as an alternative technique for processing polyimides with limited resolution and part fidelity. This study demonstrates the unprecedented 3D printing of PMDA-ODA using mask-projection stereolithography, and the preparation of high-resolution 3D structures without sacrificing bulk material properties. Synthesis of a soluble precursor polymer containing photo-crosslinkable acrylate groups enables light-induced, chemical crosslinking for spatial control in the gel state. Postprinting thermal treatment transforms the crosslinked precursor polymer to PMDA-ODA. The dimensional shrinkage is isotropic, and postprocessing preserves geometric integrity. Furthermore, large-area mask-projection scanning stereolithography demonstrates the scalability of 3D structures. These unique high-performance 3D structures offer potential in fields ranging from water filtration and gas separation to automotive and aerospace technologies.